Modulation Techniques and How They Work

Exam: 640-721 - Implementing Cisco Unified Wireless Networking Essentials (IUWNE)


Modulation is a process of variations in the signal or the tone known as carrier signal. Encoding is a process wherein the data is appended to the signal. The wireless networks use modulation to send data. With the help of radio signals it sends the encoding data. There three forms of modulated waveform are:

  • Amplitude
  • Phase
  • Frequency

Different techniques of modulation that are used by wireless networks are:

  • DSSS
  • OFDM

DSSS

802.11b devices use DSSS modulation technique to send data. The transmission signal spreads through the full spectrum of a particular frequency. While using DSSS, chip sequence is used for encoding data. Chip is used for carrier encoding and bit represents the data. Changing information from a particular format to a different format is known as encoding.

Chipping Codes

As the wireless transmission is influenced by noise interference, a series of chips are used by DSSS. If some of the signal is lost due to noise while sending of the data, it can still be understood. The chipping code process takes each data bit and then expands it into a string of bits. A chipping sequence helps to make 802.11 networks stronger against interference. Normally the rate of chipping exceeds the data rate.

Barker Code

802.11 use barker code for achieving rates anywhere between 1 to 2 Mbps. While encoding a data this code specifies using 11 chips. Radio waves can be perfectly modulated using this code. Each bit of data is coded into barker code before being modulated to DSSS.

Complementary Code Keying

There are two types of data rates. Barker code is used for lower data rates and for higher data rates CCK is used. CCK uses complementary sequences.

Date Rate Encoding Modulation
1 11 chip Barker coding Differential Binary Phase Shift Keying
2 11 chip Barker coding Differential Quadrature Phase Shift Keying
5.5-16 8 chip encoding CCKD coding Differential Quadrature Phase Shift Keying
11-128 8 chip encoding CCKD coding Differential Quadrature Phase Shift Keying

DSSS Encoding Methods

OFDM

It is used for modulation in wireless networks. It is not seen as a spread spectrum technology. It characterizes a superb data rate and strong data corruption resistance due to interference. It explains number of channels in a frequency range which are further divided into a larger number of small-bandwidth subcarriers. Though each sub carrier has a data rate that is lower, data sent is in parallel subcarriers achieving higher data rates.

MIMO

MIMO is capable of delivering data rates in excess of 100 Mbps and it uses more than one antennas for reception of signals.

There is a possibility of collision in wireless networks. Wireless devices cannot tell when a collision occurs. They can only identify after its occurrence. To avoid collisions on a wireless network, carrier sense multiple access collision avoidance (CSMA/CA) is used.


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